Charles Alford

Characterization of Si3N4/SiO2 planar lightwave circuits and ring resonators

The large refractive index contrast between silicon nitride and silicon dioxide allows silicon nitride/dioxide planar waveguides to have a small mode size and low radiation bending loss compared with doped silicon dioxide waveguides. Small waveguide bend with low radiation loss can help make small integrated planar lightwave circuits (PLCs), and also high-Q waveguide ring resonators. This presentation will talk about the design, fabrication and characterization of low loss silicon nitride/dioxide planar waveguide devices including waveguide bend, waveguide cross, and leaky mode waveguide polarizer. The key contribution of this work is the use of the lateral mode interference (LMI) 3dB splitter to accurately measure the loss of the planar lightwave circuit devices. We will also talk about the waveguide ring resonators with silicon nitride/dioxide materials. The application for photonic biochemical sensors will also be discussed.

Fabrication and measurement of wideband achromatic waveplates for the mid-infrared region using subwavelength features

Subwavelength diffractive features etched into a substrate lead to form birefringence that can produce polarization sensitive elements such as wave plates. Using etched features allows for the development of pixelated devices to be used in conjunction with focal plane arrays in polarimetric imaging systems. Form birefringence exhibits dispersion that can be advantageous to the design of wave plates with an achromatic response. Taking advantage of this dispersion, diffractive wave plates with good achromatic characteristics can be designed for the 2- to 5-µm spectral region. Previous work in this area has produced good results over a subset of this wavelength band, but designing for this extended band is particularly challenging. The fabrication processes for the subwavelength features will be discussed and fabricated devices with a measured average phase retardation of 80.6 deg and rms variation of 9.41 deg will be presented.

Optical AND and NOT gates at 40 Gbps using electro-absorption modulator/photodiode Pairs

We demonstrate an optical gate architecture using electro-absorption modulator/photodiode pairs to perform AND and NOT functions. Optical bandwidth for both gates reach 40 GHz. Also shown are AND gate waveforms at 40 Gbps.

Integrated Guided-Wave Photodiode Using Through-Absorber Quantum-Well-Intermixing

A high-speed, high-saturation power photodiode compatible with a relatively simple monolithic integration process is described. The detector is comprised of an intrinsic bulk absorption layer, an electron drift region, and a field termination layer, and is grown above a main waveguide core comprised of a number of quantum wells, which are used as the active region of a phase modulator. Through-absorber quantum-well-intermixing is used to blue-shift the bandedge of the underlying quantum wells, reducing the optical losses of that material. The detectors demonstrate quantum efficiency, input saturation power, and 3-dB bandwidth of 50 GHz.

Optical logic gates using interconnected photodiodes and electro-absorption modulators.

We demonstrate an optical gate architecture with optical isolation between input and output using interconnected PD-EAMs to perform AND and NOT functions. Waveforms for 10 Gbps AND and 40 Gbps NOT gates are shown.

Power maximization in III–V sub-millimeter, radial front contacted cells for thin micro-concentrators

Sub millimeter scale solar cells coupled with medium concentration lenses can reduce the balance of system costs of concentrating photovoltaics by creating thin and highly efficient concentrators with relaxed tracking requirements. This paper shows the design, fabrication, simulation, and testing of micro-sized photovoltaics that have unique perimeter front contacts outside the optical collection area. The design of the device considered the need for low resistance current carrying layers while minimizing optical losses. The paper also shows the successful fabrication of InGaAs cells as well as of GaAs cells transferred onto silicon substrates. The simulations and experimental measurements show that small cells of this type 1) suffered from slightly lower voltage levels caused by proportionally larger dark currents 2) peaked their efficiencies at higher concentration levels compared to larger ones 3) performed better overall.

Bonded InGaAs cells for microsystems enabled photovoltaics

InGaAs solar cells bonded to a Si substrate are demonstrated. These cells are 160 μm to 1300 μm in diameter and designed for integration in microsystems enabled photovoltaic systems. When compared to devices fabricated on substrate no degradation of cell performance was observed due to bonding. Additionally, the short circuit current for the cells correlated well with simulations indicating a low loss optical path through the bonding interface.

Layer disordering and doping compensation of an intersubband AlGaN/AlN superlattice by silicon implantation

Layer disordering and doping compensation of an Al0.028Ga0.972N/AlN superlattice by implantation are demonstrated. The as-grown sample exhibits intersubband absorption at ∼1.56 μm which is modified when subject to a silicon implantation. After implantation, the intersubband absorption decreases and shifts to longer wavelengths. Also, with increasing implant dose, the intersubband absorption decreases. It is shown that both layer disordering of the heterointerfaces and doping compensation from the vacancies produced during the implantation cause the changes in the intersubband absorption. Such a method is useful for removing absorption in spatially defined areas of III-nitride optoelectronic devices by, for example, creating low-loss optical waveguides monolithically that can be integrated with as-grown areas operating as electro-absorption intersubband modulators.

Resistance considerations for stacked small multi-junction photovoltaic cells

In this paper we propose a stacked multi-junction solar cell design that allows the intimate contact of the individual cells while maintaining low resistive losses. The cell design is presented using an InGaP and GaAs multi-junction cell as an illustrative example. However, the methodologies presented in this paper can be applied to other III-V cell types including InGaAs and InGaAsP cells. The main benefits of the design come from making small cells, on the order of 2×10-3 cm2. Simulations showed that series resistances should be kept to less than 5 Ω for devices up to 400 μm in diameter to keep resistance power losses to less than 1%. Low resistance AuBe/Ni/Au ohmic contacts to n-type InGaP are also demonstrated with contact resistivity of 5×10-6 Ωcm-2 when annealed at 420°C.

Enhanced frequency response in monolithically integrated coupled cavity lasers and electro-absorption modulator

We present the bandwidth enhancement of an EAM monolithically integrated with two mutually injection-locked lasers. An improvement in the modulation efficiency and bandwidth are shown with mutual injection locking.